NAT

NAT stands for Network Address Translation.

It solves an important problem:

How can many private devices access the internet using one public IP?

Why do we need NAT?

At home, many devices may use private IPs:

Laptop : 192.168.1.10
Phone  : 192.168.1.11
TV     : 192.168.1.12

But your internet provider usually gives your router one public IP.

So when all these devices access the internet, the outside world usually sees:

Router Public IP

not each private device IP.

NAT makes this possible.

What does NAT do?

When your laptop sends a request to the internet:

192.168.1.10 → google.com

the router changes the source address.

Before NAT:
Source IP = 192.168.1.10

After NAT:
Source IP = Router's Public IP

When the reply comes back, the router remembers which private device made the request and forwards the reply to that device.

NAT Translation Table

The router keeps a temporary table.

Example:

Private Device        Public Mapping
192.168.1.10:51510 → 49.x.x.x:60001
192.168.1.11:51511 → 49.x.x.x:60002

This helps the router know where to send replies.

Simple Flow

Laptop
192.168.1.10

↓

Router performs NAT
Private IP becomes Public IP

↓

Internet Server

Reply:

Internet Server

↓

Router checks NAT table

↓

Laptop
192.168.1.10

Interview Answer

If an interviewer asks:

Why is NAT needed?

You can answer:

NAT allows multiple private devices inside a local network to access the internet using a shared public IP address. The router translates private source IPs into its public IP and keeps a translation table so replies can be sent back to the correct internal device.